1. Field of the Invention
This invention pertains generally to the field of navigation systems such as, but not limited to, aircraft navigation systems.
2. Description of the Related Art
The continuing growth of aviation has placed increasing demands on airspace capacity and emphasizes the need for the best use of the available airspace. These factors, along with the accuracy of modern aviation navigation systems and the requirement for increased operational efficiency in terms of direct routings and track-keeping accuracy, have resulted in the concept of “Required Navigation Performance” (“RNP”) standards—statements of the navigation performance accuracy necessary for operation within a defined airspace. Some of these standards appear in an Advisory Circular (“AC”) published by the Federal Aviation Administration (“FAA”) and in a Document (“DO”) published by the Radio Technical Commission for Aeronautics (“RTCA”). For example, the FAA has published AC 120-29A entitled “Criteria for Approval of Category I and Category II Weather Minima for Approach,” and the RCTA has published DO-236B entitled “Minimum Aviation System Performance Standards: Required Navigation Performance for Area Navigation.”
RNP may include performance and functional requirements indicated by the RNP type. The RNP type defines the total system error (“TSE”) that may be allowed in lateral and longitudinal dimensions within a particular airspace, where the TSE takes into account path definition errors (“PDE”), navigation system errors (“NSE”), and flight technical errors (“FTE”). The RNP type is used to specify navigation requirements for the airspace or a navigation system that provides a specified level of accuracy defined by a lateral area of confined airspace in which an RNP-certified aircraft operates. For example, an RNP 0.3 level requires a normal navigational accuracy of 0.3 nautical miles (“NM”) at least 95 percent of the total flight time; similarly, an RNP 0.1 level requires a normal navigational accuracy of 0.1 NM at least 95 percent of the total flight time. Hence, a lower level of RNP means a greater level of navigational accuracy. With the availability of satellite navigation systems such as the Global Positioning System (“GPS”), future airspace systems may rely heavily on GPS-based navigation to which stringent standards for NSE may be applied. An instrument approach procedure (“IAP”) specifying an RNP level of less than RNP 0.3 or lower than the standard minima at airports not equipped with Instrument Landing Systems (“ILS”) airports may require a method to detect GPS position errors before the 6.2 second time-to-alert standard that currently exists. A similar requirement may be made for manufacturers and/or end-users wishing to take advantage of the standards published in RTCA DO-315B entitled “Minimum Aviation System Performance Standards (MASPS) for Enhanced Vision Systems, Synthetic Vision Systems, Combined Vision Systems and Enhanced Flight Vision Systems.”
One way to achieve a greater level of navigational accuracy is to reduce the TSE. Because the TSE may be comprised of the PDE, NSE, and FTE, a reduction of any of these—such as the NSE—could reduce the TSE which, in turn, could produce a greater level of navigational accuracy. A reduction of NSE could be obtained by monitoring and/or augmenting navigation data provided by one or more navigation systems with an independent source of navigation data. Such independent source could be used to validate the navigation system navigation data by confirming the navigation system navigation data falls within lower desired tolerances commensurate with a lower NSE. Moreover, the confirmation of valid navigation may permit a reduction of a minimum decision altitude (“MDA”) and/or decision height (“DH”) of an IAP performed during an approach-for-landing phase of flight, where one such procedure may be a Localizer Performance with Vertical Guidance (“LPV”) IAP or any IAP with vertical guidance requiring higher integrity.
The independent source could also be used to generate altitude data and/or height data. The use of an independent source to generate altitude data and/or height data is disclosed in U.S. Pat. No. 8,321,074 entitled “Altitude Data Generation System, Device, and Method,” which is hereby incorporated by reference in its entirety.